1. Field of the Invention
The present invention relates to an add-drop multiplexing (ADM) apparatus for dropping a low-order signal from a high-order signal, or adding a low-order signal to a high-order signal.
The ADM apparatus is used for an SONET (synchronous optical network) and an SDH (synchronous digital hierarchy), to carry out cross-connections by dropping a low-order signal from a high-order signal and adding a low-order signal to a high-order signal. It is required to improve the efficiency of the ADM apparatus and reduce the cost thereof.
2. Description of the Related Art
To transfer a signal between high-order networks or between a high-order network and a low-order network according to a prior art, each network must have an ADM apparatus to drop a low-order signal from the network, secure a path for the dropped signal, and add the dropped signal to the other network. Each ADM apparatus must have a low-order interface to be connected to the ADM apparatus of the other network, as well as path setting units whose outputs are selected according to selection data.
This arrangement of the prior art increases the scale and cost of a network system because each network must employ an ADM apparatus when connecting it to another network and because each ADM apparatus must have a low-order interface to be connected to another ADM apparatus.
The prior art involves intricate path setting and must have a device for generating an unequipped signal for a path that is not ready.
The prior art arranges an alarm detector for each signal level to complicate the structure of the ADM apparatus.